The present invention relates generally to the field of spinal implant systems, and particularly systems that employ elongated spinal implants, such as rod and plates, connected at various locations along the spinal column. More particularly, the invention concerns a connection assembly that provides variable angle and variable height adjustability to the elongated spinal implant relative to a bone fastener engaged to the spine.
Several spinal fixation systems have been developed for use in correcting and stabilizing sections of the spinal column and facilitating spinal fusion. In one such system, a bendable elongated spinal implant, such as a rod, is longitudinally disposed adjacent the vertebral column and then secured to various vertebrae along the length of the column by way of a number of bone fasteners or fixation elements. A variety of bone fasteners can be utilized, such as hooks or bone screws, which are configured to engage specific portions of a vertebra.
An example of one such system is the TSRH.RTM. Spinal System of Sofamor Danek Group, Inc. In this system, various hooks and bone screws are engaged to a spinal rod by way of eyebolts. In early versions of the TSRH.RTM. Spinal System, the vertebral hooks and bone screws were attached to the spinal rod at a fixed orientation, usually projecting perpendicularly below the rod. At the time, the TSRH.RTM. Spinal System presented a significant advance over prior systems in its VERSATILITY, strength of fixation, and ease of implantation.
However, one drawback faced by the original TSRH.RTM. Spinal System, as well as the other prevalent fixation systems, was that a surgeon was required to make significant adjustments to the contour of the bendable rod so that the bone fasteners could solidly engage the vertebral bodies. What was needed, then, was a bone fastener that could be connected to the spinal rod at a variable angle. In order to address this need, the TSRH.RTM. Variable Angle Screw was developed, as described in U.S. Pat. No. 5,261,909. This Variable Angle Screw utilized the same TSRH.RTM. eyebolt to achieve a connection to a spinal rod. In addition, the Variable Angle system incorporated a washer that fit over the eyebolt, engaged the spinal rod within a groove in one surface of the washer, and provided a radially splined surface facing the bone fastener. The bone fastener had a complementary splined surface so that the fastener could be situated at variable angular orientations relative to the spinal rod. A nut threaded onto the post of the eyebolt clamped all the components together to complete the assembly.
The Variable Angle Screw system of the '909 Patent presented a significant advance over prior rod-based implant systems. The system of the '909 Patent was relatively compact and required a minimal number of parts yet was able to accomplish a solid fixation of the bone fastener to the rod at a wide range of angular orientations. One drawback of the system was that the eyebolt-nut combination required side-tightening of the nut to clamp the system together. This side-tightening aspect required a larger surgical site about the spine so that a wrench could be manipulated. To address this difficulty, a top-tightening assembly was developed as disclosed in U.S. Pat. No. 5,282,801. The clamp assembly depicted in the '801 Patent replaced the eyebolt and nut with a clamp body having a T-bar against which the head of the variable angle bone fastener was clamped. In addition, while the original TSRH.RTM. System relied upon tightening a nut against the variable angle bone screw, the top-tightening approach of the '801 Patent utilized a set screw that acted against the spinal rod to push the spinal rod into the interlocking washer, and ultimately against a complementary spline face of the variable angle screw. With this system, the variable angle capability was retained, while a top-tightening feature was added.
With the addition of the top-tightening capability, the more recent TSRH.RTM. Spinal System has provided surgeons with a great deal of flexibility in the placement and orientation of bone fasteners, such as hooks and screws, relative to a spinal rod. The Variable Angle components greatly reduce the need to manipulate and bend the spinal rod to conform to the patient's anatomy. Even with the great improvements presented by the TSRH.RTM. Spinal System, a certain amount of shaping or contouring of the spinal rod has still been required. Specifically, the rod must be shaped so that at the point of attachment of the bone fastener, the rod is the same distance from the vertebral body as the splined or interdigitating portion of the bone fastener. This vertical or height alignment is necessary so that the variable angle components are properly aligned for accurate connection when the assembly is clamped together. Thus, the spinal surgeon still has to spend a certain amount of time shaping the spinal rods during the surgery so that the fixation system can be properly implanted.
In order to address this difficulty, later systems were developed that provided for a certain degree of vertical adjustability. By vertical or height adjustability, it is meant adjustment along the length of the bone fastener. Adjustment in this dimension allows the rod to be situated at varying distances from the spine, or oriented with a pre-set contour regardless of the location of fastener.
An adaptation of the original variable angle screw concept of the '909 Patent is presented in U.S. Pat. No. 5,611,800. This system retained the yoke configuration of the bone screw in the '909 Patent, but added a multi-faceted connecting feature on both surfaces of the yoke. While the '800 Patent system added height adjustability it did so at the cost of a more complicated connector structure with four specially machined interdigitating surfaces.
Another approach has been suggested in U.S. Pat. No. 5,643,263. The connection assembly in the '263 Patent uses a Schanz-type bone screw rather than the yoke bone screw of the '909 Patent.
Thus, the screw described in the '263 Patent includes an elongated smooth shank portion. The connection assembly also adds a second washer disposed between the original washer and the smooth shank of the bone screw. The interdigitating feature exists between the contacting faces of the adjacent washers. The variable height is accomplished by a groove provided in the opposite surface of the additional washer that allows the connection assembly to slide along the shank of the bone screw until it is finally clamped together by a set screw.
While the connection assembly shown in the '263 Patent goes a step further toward an easy-to-implant variable angle, variable height assembly, it too has left some room for improvement. For example, the connector assembly of the '263 Patent requires an additional washer that adds to the number of components that must be manipulated by the surgeon. In addition, the added washer increases the lateral profile of the implant assembly. In certain regions of the spine, such as the thoracic and cervical regions, there is limited space transverse to the vertebral bodies. An optimal spinal implant system that has universal applicability should have as small a lateral profile as possible.